The treatment of a cancer is one of applications of radiation beam. Recently, particle beam treatment in which a particle beam such as a proton beam or a carbon beam is radiated on the cancer cell has been attracted attention. First of all, the characteristic of a particle beam radiation in which the particle beam is radiated to kill a cancer cell will be described. In a case where various kinds of radiation beams are radiated on a human body, the dose distribution of the radiation beam in the human body changes as shown in FIG. 18. As shown in FIG. 18, among various kinds of radiations, a photon beam such as an X-ray or a gamma ray has a relative dose which becomes maximum in a portion close to the surface of the body, and is decreased as the depth from the surface of the body is increased. On the other hand, a particle beam, such as a proton beam or a carbon beam, has a relative dose which has a peak value at a position where the beam stops at a deep portion from the surface of the body, that is, immediately before the range of the particle beam. This peak value is called the Bragg Peak (BP).
Particle beam cancer treatment is such that this Bragg peak BP is radiated to a tumor formed in a human organ and the treatment of the cancer is performed. In addition to the cancer, it can also be used for a case where a deep portion of a body is treated. A region to be treated, including a tumor, is generally called a target volume (TV). The position of the Bragg peak BP is determined by the energy of an radiated particle beam, and as the energy of the particle beam becomes high, the Bragg peak BP is formed at a deep position. In the particle beam treatment, it is necessary that the particle beam is made to have a uniform dose distribution over the whole of the target volume to be irradiated. In order to give the Bragg peak BP to the whole region of the target volume, “spread of an irradiation volume” of the particle beam is performed.
This “spread of an irradiation volume” is performed in three directions of an X-axis, a Y-axis and a Z axis perpendicular to each other. When the irradiation direction of the particle beam is set to be the direction of the Z-axis, “spread of an irradiation volume” is first performed so as to spread the irradiation field in the X-axis and Y-axis directions, and since the irradiation field spread is performed in the lateral direction perpendicular to the depth direction, it is generally called the irradiation field spread. The second “spread of an irradiation volume” is such that the irradiation volume spread is performed in the Z-axis, and it is called the irradiation volume spread in the depth direction.
The irradiation volume spread in the depth direction is performed to spread the Bragg peak BP, which is in the irradiation direction of the particle beam, to the depth direction since the width of the Bragg peak BP in the irradiation direction of the particle beam is narrow as compared with the extent of the target volume in the depth direction. On the other hand, the irradiation field spread in the lateral direction is performed to spread the irradiation field in the Bragg peak BP in the direction perpendicular to the irradiation direction since the diameter of the particle beam, which is accelerated by an accelerator generally, is smaller than the size of the target volume in the direction perpendicular to the irradiation direction. With respect to the irradiation volume spread in the depth direction and the irradiation field spread in the lateral direction, various kinds of methods have been proposed so far. Recently, Scanning Irradiation method has attracted attention.
In Scanning Irradiation method, as an irradiation field spread method in a lateral direction, there is a method in which a deflection electromagnet provided at the upstream portion of a particle beam irradiation part of a particle beam treatment device is used to scan the particle beam which is formed at a pencil beam having a small diameter in the XY plane, and the irradiation position of the particle beam is moved with the lapse of time to obtain a wide irradiation field. In this method, a uniform dose distribution can be obtained by suitably overlapping adjacent irradiation spots of small diameter pencil beams. Scanning methods of a pencil beam include a raster method of performing scanning continuously with respect to time, and a spot method of performing a step-like scanning with respect to time.
As the irradiation volume spread method in the depth direction, there is a method in which the energy of the particle beam itself which is radiated from a particle beam treatment device is controlled. In this method, the energy of the particle beam is controlled by changing the acceleration energy of an accelerator which accelerates the particle beam, or the energy of the particle beam is changed by inserting a tool called a range shifter so as to cross the particle beam. There is also a method in which both the control by the accelerator and the range filter are used.
In the irradiation volume spread method in the depth direction, the particle beam is made to be the beam having the energy of specified intensity, after one of irradiation layers of a target volume TV is irradiated with the Bragg peak BP with a uniform dose, the energy of the particle beam is changed, and next irradiation layer of the target volume TV is irradiated with the Bragg peak BP. Such operation is repeated plural times so as for plural irradiation layers to be irradiated with the Bragg peak BP of the particle beam. Consequently, the spread-out Bragg peak SOBP having a desired width in the beam irradiation direction can be obtained.
A particle beam irradiation method in which the irradiation field spread method in lateral direction and the irradiation volume spread method in depth direction are combined is the Scanning Irradiation method.
In the Scanning Irradiation method, the target irradiation dose at an irradiation position is set by a treatment plan. The shape of irradiation field and target irradiation dose for each irradiation position depends on patients. Two kinds of dose are defined, that is, physical dose and biological dose (also referred to as biology dose and effective dose). Physical dose is energy per mass which is applied to a part having a target, and its unit is gray (Gy). On the other hand, biology dose is a value which is determined based on the physical dose in consideration of biological influence to a cell, and its unit is gray equivalent (GyE). The biology dose is defined by the condition, for example, the dose which is equivalent to the irradiation dose by cobalt 60 which makes the survival ratio of cell to be 10%. In particle beam treatment, prescription dose is defined by the biology dose. The aim of the irradiation volume spread is to make uniform the irradiation effect, and targeted dose for every patient is defined by biology dose distribution. On the other hand, by the dose monitor which is used for measuring the administration dose, the biological effect can not be measured, therefore dose calibration is performed by using the physical dose.
In particle beam treatment, it is necessary for a biology dose distribution in a target volume TV of a particle beam to be actually irradiated according to the distribution which is set in treatment plan. On the other hand, there is not any method to measure directly the biology dose in the target volume TV during irradiation. Further, with respect to outside of an affected part, only the physical dose can be measured during irradiation. Accordingly, irradiation should be applied so as to make the biology dose in a target volume TV to be a value which is set by a treatment plan while controlling the physical dose.
Therefore, the step of dose calibration is performed before irradiation, however, in the prior arts, only at one point of center of SOBP in the depth-direction distribution of biology dose, dose calibration is performed. For example, in Patent Document 1, it is disclosed such that “the target is split into a plurality of layers, and an irradiation amount for every layer is determined”. In Patent Document 2, it is disclosed such that “the target is split into a plurality of layers, and an irradiation amount for each layer is determined so as to be uniform”.
According to the technology described in Patent Document 1 or Patent Document 2, since the change of Bragg curves in the depth direction is steep, a large error is caused by a slight position error with respect to the position for setting a dose meter. As the technology for solving the above-mentioned problems, in Patent Document 3, the technology for reducing the position error by forming a flat area in a peak of Bragg curves.
Prior Art References
Patent Document
    Patent Document 1: Patent Application Laid-Open No. 2004-358237    Patent Document 2: Patent Application Laid-Open No. 10-314323    Patent Document 3: National Publication of International Patent Application No.2009-139043A1